The present invention relates generally to channel estimation, and more particularly, to improving the accuracy of scattering object characterizations used to determine channel estimates.
In a wireless communication system, objects (e.g. buildings, hills, etc.) in the environment, referred to herein as scattering objects, reflect a transmitted signal. The reflections arrive at a receiver from different directions and with different path delays. The reflections or multi-paths can be characterized by a path delay and a complex delay coefficient. Typically, a scattering object is characterized by complex delay coefficients that show fast temporal variation due to the mobility of the vehicle, while the corresponding path delays are relatively constant over a large number of transmission intervals.
Channel estimation is the process of characterizing the effect of the radio channel on the transmitted signal. Channel estimates approximating the effect of a recent propagation channel on the transmitted signal may be used for interference cancellation, diversity combining, ML detection, and other purposes. Channel estimates may also be used to provide a transmitter with knowledge of a future transmission propagation channel. The U.S. patents to Applicant Dent listed below are incorporated by reference herein, and individually or jointly disclose the benefits that may be obtained in a mobile communications system when a transmitter, e.g., a fixed base station, obtains knowledge of the characteristics of the transmission propagation channels, e.g., the downlink propagation channels.                U.S. Pat. No. 6,996,375 titled “Transmit diversity and separating multiple loopback signals;”        U.S. Pat. No. 6,996,380 titled “Communications system employing transmit macrodiversity;”        U.S. Pat. No. 7,197,282, titled “Mobile Station loopback signal processing;” and        U.S. Pat. No. 7,224,942 titled “Communications system employing non-polluting pilot codes.”Methods for providing knowledge of a past propagation channel, described in the above and other known art, include providing feedback signals from the mobile stations, looping back signals from the mobile stations, and using the same frequency for the downlink as for the uplink in a so-called Time-Division-Duplex (TDD) system.        
TDD operation is however not always appropriate, particularly when the communications system operates over long ranges, making the concept of simultaneity in different places moot. Also, including deliberate loopback or feedback signals in transmissions from mobile stations to fixed base stations may require a large amount of uplink capacity when speeds are high. Therefore, there is interest in methods that enable a transmitter to determine the transmission channel in advance based on normally received traffic, even when the reception frequency band is different than the transmission frequency band. Extrapolating channel information that has been determined by analyzing signals over a reception frequency band, e.g., a 20 MHz bandwidth centered at one center frequency, to channel information for a transmission frequency band centered at another center frequency separated from the reception center frequency by, e.g., 200 MHz, places challenging requirements on the accuracy of the channel model and estimates of the model parameters. In fact, extrapolating channel parameters to a different frequency band places the greatest requirements on the accuracy of the scattering object model used to represent the propagation channel environment. Improved accuracy would however be welcomed for other purposes too, such as for better data decoding, position determination, etc.
It is generally assumed that estimates of radio propagation channels are limited by a certain “coherence bandwidth,” meaning that signals separated by more than the coherence bandwidth likely have no correlation between their propagation channels. Similarly, it is generally assumed that estimates of radio propagation channels are limited by a certain “coherence time,” meaning that there is no expected coherence between channel values taken at times separated by more than the coherence time limit. However, the inventors postulate that current coherence bandwidth and time limits are not hard limits and instead are more a symptom of the channel model inaccuracies. Thus, the inventors propose that a more complex and more accurate channel model will increase the coherence bandwidth and coherence time limits, and perhaps even eliminate the perception of a limited coherence bandwidth and time. In environments characterized by a large number of physically small and randomly distributed scattering objects, such as leaves on trees, it may still not be possible to build a channel model of adequate complexity and accuracy to overcome the perception of a coherence bandwidth limit. However, the basic postulate may be valid in other environments characterized by a reasonable number of large scattering objects.
While researching the above issues, the applicants filed the following related U.S. patent applications, which are hereby incorporated by reference herein:                U.S. patent application Ser. No. 12/478,473 titled “Improved Mobile Radio Channel Estimation,” which describes a “delay-first” approach to characterizing each scattering object by its path delay and Doppler shift. Several adaptations and improvements to the Prony algorithm were combined therein for determining the path delays and Doppler shifts. The Prony algorithm was adapted first to analyze a radio channel in order to determine scattering path delays. Then the amplitude versus time of each delayed ray was further analyzed by a second adaptation of the Prony algorithm to resolve different Doppler shifts for each path delay.        U.S. patent application Ser. No. 12/478,520 titled “Continuous Sequential Scatterer Estimation,” which discloses that a Doppler shift is in fact simply another measure of rate-of-change of delay, e.g., relative velocity, and that a useful scattering object characterization comprises path delay and rate-of-change of delay, rather than path delay and Doppler shift. Thus, after finding different path delays and Doppler shifts using the Prony method, the Doppler shifts were translated to rate of change of delay values, and then a Kalman algorithm was used to track the path delay and its derivative while using the Prony algorithm to search for new scattering objects not already being tracked by the Kalman filter.        U.S. patent application Ser. No. 12/478,564 titled “Channel Extrapolation from one Frequency and Time to Another,” which extrapolates propagation channel information from one time and frequency, e.g., a reception time and frequency or frequency band, to another time and frequency, e.g., a transmission time and frequency or frequency band. This application places the toughest accuracy requirements on scattering parameter estimation.        
The above-referenced applications generally assume that the Doppler shift/rate-of-change-of-delay is constant over a received signal bandwidth. When signal bandwidths are small, so that there is little difference between a highest signal frequency and a lowest signal frequency, this assumption is generally accurate. Thus, for narrowband signals, translation from Doppler shift to rate-of-change-of-delay can be made accurately by just using the center frequency. However, wireless communications continue to demand, obtain, and use more and more bandwidth in the quest for higher data rates. For wideband signals, a given rate-of-change-of-delay does not translate exactly to the same Doppler shift at the edges of the bandwidth. For very wideband applications, this error can hinder the achievement of the most ambitious accuracy goals, such as those required for channel extrapolation to different frequency bands or widely separated times. Therefore, a more accurate method of resolving a radio channel into the scattering parameters of path delay and Doppler shift (or rate-of-change-of-delay) is required when using very wideband signals.